The prostaglandins are a well known group of compounds which have been shown to have a variety of valuable pharmacological activities. Recently, prostacyclin (PGI.sub.2) has been discovered and shown to have strong platelet aggregation inhibitory activity; unfortunately, prostacyclin is rather unstable and is rapidly hydrolized to the stable, but less active, 6-oxoprostaglandin F.sub.1.alpha.. We have now discovered a series of prostacyclin derivatives, whose activities are comparable with those of prostacyclin and prostaglandin E.sub.1 (PGE.sub.1), but which are much more stable than prostacyclin and PGE.sub.1.